We're here, so there was some advantage to our irrationality. Hating outsiders helps our children survive for example.
I'd argue a lot of our best inventions required a certain amount of irrationality to create.
I think you may be begging the question. You might be right that we're here because of our irrationality, but using your conclusion as its own argument isn't very convincing.
No, all you can infer from the fact that we're still here is that it's possible to succeed while being irrational. It tells you nothing about whether we succeeded despite or because of it.